Telecommunication service providers are continually seeking ways to upgrade their operational capabilities without a substantial cost and/or power penalty to existing equipment infrastructures. In particular, service providers have sought ways to augment their legacy remote terminals (RTs) with digital data capability without impacting existing POTS equipment. A fundamental problem is the fact that the remote terminals were originally configured prior to the advent of DSL services, so that in many instances there is a housing space limitation as far as adding new circuits is concerned. Moreover, DSL service operates under a different set of parameters than does POTS and, in the case of remotely located equipment cabinets, requires a battery back-up package (for lifeline service) in the event of a local power outage.
One conventional architecture for providing both POTS and DSL service in a remote terminal is diagrammatically illustrated in FIG. 1, as comprising a POTS/DSL splitter 10 installed in the subscriber loop. The splitter serves to spectrally separate (as shown in the spectral diagram of FIG. 2) the relatively low frequency band used for POTS voice and ringing signals (on the order of four KHz) from an upper frequency band (on the order of from 25 KHz to 1.1 MHz) used for DSL signals. Customarily, the POTS line conveys a −48 VDC voltage differential for powering the POTS terminal equipment, and includes a ringing relay for ringing the telephone. A subscriber line interface circuit (SLIC) 20 interfaces the POTS channel with voice band equipment (codec), while DSL driver circuitry 30 interfaces the DSL channel with digital band terminal equipment. In order to isolate the lower voltage (+/−+12 VDC) used by the digital channel from the −48 VDC of the POTS line, the high pass port of the band splitter 10 is usually transformer-coupled with the DSL link, as shown at 40.
A fundamental shortcoming of the band-splitter approach of FIG. 1 is its size and complexity, which results from what is essentially a DSL add on. In addition to the fact that separate drivers are used for the POTS and DSL channels, the band splitter is usually a high order device (e.g., a sixth order filter), in order to achieve the required channel separation (65-70 dB). An alternative approach is to use a lower order filter (such as a second order filter); however, doing so requires careful design of the POTS line driver circuitry to accommodate signal leakage (shown in the spectral diagram of FIG. 3). This results from the lower order filter implementation.
Another proposal, diagrammatically illustrated in FIG. 4, is to construct a broadband ringing SLIC, that is capable of handling both DSL and POTS signaling simultaneously. A principal drawback to this architecture is the need to accommodate a multitude of requirements in a single design. To accommodate maximum distance POTS signaling, including ringing, balanced line signaling is required. Without transformer coupling, protection resistors must also be part of the circuit, and a significant portion of the DSL signal is lost across these components. Also, DC offset is used for detection of off-hook on long subscriber loops. When all of these requirements are taken into account, what results is a broadband circuit design that needs a line voltage on the order of −180 VDC. Thus, the circuit is complex, expensive and suffers from substantial power consumption.